1. Field of the Invention
The present invention relates to an optical wavelength converter device for converting a fundamental to a second harmonic having a wavelength which is 1/2 of the wavelength of the fundamental or to a third harmonic having a wavelength which is 1/3 of the wavelength of the fundamental, or to an optical wavelength converter device for converting two fundamentals of different wavelength to two kinds of light each having an angular frequency equivalent to the sum or difference of the angular frequencies of the two fundamentals, and more particularly to an optical wavelength converter device employing an organic nonlinear optical material for obtaining high wavelength conversion efficiency through a resonant effect.
2. Description of the Prior Art
Various attempts have heretofore been made for converting the wavelength of a laser beam into a shorter wavelength based on the generation of a second harmonic by a nonlinear optical material. One example of an optical wavelength converter device for effecting such laser wavelength conversion is a bulk crystal type converter device as disclosed, for example, in "Introduction to Optical Electronics" written by A. Yariv and translated by Kunio Tada and Takeshi Kamiya (published by Maruzen K. K.), pages 200-204. This optical wavelength converter device relies upon the birefringence of a crystal in order to meet phase matching conditions. Therefore, any material which does not exhibit birefringence or exhibits only small birefringence cannot be employed even if it has high nonlinearity.
A fiber type optical wavelength converter device has been proposed to solve the above problem. The optical wavelength converter device of this type is in the form of an optical fiber comprising a core made of nonlinear optical material surrounded by cladding. One example of such an optical fiber is shown in the bulletin Vol. 3, No. 2, of the microoptics research group of a gathering of the applied physics society, pages 28-32. Recently, many efforts are directed to the study of a fiber type optical wavelength converter device since it can easily gain matching between a fundamental and a second harmonic. Another known optical wavelength converter device includes a two-dimensional optical waveguide made of a nonlinear optical material disposed between two substrates serving as cladding, as disclosed in Japanese patent application Nos. 61-159292 and 61-159293 filed by the present applicant. There is also known an optical wavelength converter device comprising a three-dimensional optical waveguide of a nonlinear optical material embedded in a glass substrate for radiating a second harmonic into the glass substrates. These optical wavelength converter devices with the optical waveguide offer the same advantage as that of the fiber type optical wavelength converter device.
Further, it is described in detail in copending Jap. patent application No. 63(1988)-72752 that the fiber type optical wavelength converter device generates a sum-frequency wave and a difference-frequency wave as well. The generator of the sum-frequency wave and the difference-frequency wave in a waveguide type optical wavelength converter device is disclosed in copending Japanese patent application No. 63(1988)-72753. It is also possible to generate a third harmonic by use of 3-dimensional non-linearity of the optical material.
Attention has recently been directed in the art to an organic nonlinear optical material which can produce a second harmonic through a resonant effect, as disclosed, for example, in "Organic nonlinear optical materials" supervised by Masao Kato and Hachiro Nakanishi (published by C.M.C. Co. in 1985). The principles of the generation of a second harmonic through the resonant effect will briefly be described below. As is well known, assuming that the frequencies of a fundamental and a second harmonic are indicated by .omega..sub.0, 2.omega..sub.0, and the frequency of light absorbed by a nonlinear optical material is .omega., a nonlinearity constant d is given as follows: ##EQU1## where A and B are constants. Therefore, if the optical wavelength .lambda.max at which the light absorption coefficient is maximum is close to at least one of the fundamental wavelength and the second harmonic wavelength, then a resonant effect is produced to greatly increase the nonlinearity constant d.
By employing an organic nonlinear optical material which can generate a second harmonic through such a resonant effect, it is possible to gain wavelength conversion efficiency which is higher, by two digits or more, than possible if a second harmonic would be generated without any resonant effect.
Similarly, in the generation of the sum- or difference-wavelength wave and in the generation of the third harmonic, a similar phenomenon is expected. For example, in case of the generation of the sum- or difference-frequency wave, the non-linearity constant d would be ##EQU2## where .omega..sub.1, .omega..sub.2 and .omega..sub.3 are frequencies of the two fundamentals and the sum-frequency wave, respectively, and .omega. is the frequency of light absorbed by a non-linear optical material. Therefore, if the optical wavelength .lambda. max at which the light absorption is maximum is close to at least one of the wavelength of the two fundamentals and the sum- or difference-frequency wave, a resonant effect is produced to greatly increase the non-linearity constant d.
Thus, in case of the sum- or difference-frequency wave and the third harmonic also, it is possible to gain wavelength conversion efficiency which is higher, by two digits or more, than possible if the wavelength is converted without any resonant effect.
The phenomenon described above has only been confirmed on a thin-film organic nonlinear optical material. Any actual optical wavelength converter device capable of converting optical wavelengths highly efficiently by utilizing the resonant effect is not available so far. The reasons for this in case of the conversion of a fundamental to a second harmonic are as follows:
In order to produce a resonant effect, as described above, an organic nonlinear optical material which can well absorb light having a wavelength close to that of a fundamental or a second harmonic is employed. Where an organic nonlinear optical material in the form of powder or a bulk crystal, for example, which can well absorb light having a wavelength close to that of a second harmonic is employed, since the second harmonic that has been converted in wavelength highly efficiently is absorbed by the organic nonlinear optical material itself, the amount of light of the second harmonic which can actually be extracted from the optical wavelength converter device for use is highly limited. For quite the same reason, in case of the generation of the sum- or difference-frequency or the third harmonic also, there have not been provided a device which is capable of converting a wavelength with high efficiency.